Inertial reference systems (IRS) are used to provide inertial navigation data for aircraft navigation. Various types of sensors can be mounted on the aircraft. An airborne sensor that is used for targeting and localization using the navigational system of the host aircraft must be calibrated to the inertial references of the aircraft. An inertial navigation unit within an aircraft provides attitude information (i.e., roll, pitch and heading) in addition to position and velocity information. Long baseline sensors report detections with respect to their coordinate axes (e.g., angle-of-arrival at the antenna). The orientation of the sensor's coordinate axes with respect to the inertial navigation unit's attitude angles must be determined in order to accurately convert sensor detections to recognized earth fixed coordinates (e.g., latitude, longitude, and altitude).
For sensors that use the angle-of-arrival of a received signal for location determination (e.g., as performed by radars and phase interferometers), the location accuracy is limited by the accuracy of the angle-of-arrival measurements. Recent technology improvements in inertial navigation units (INU) have provided the opportunity for dramatic improvements in the accuracy of sensor angle measurements. However, these improvements can only be realized if the INU coordinate frame can be precisely aligned with the sensor's coordinate frame. This is often very difficult to accomplish with traditional alignment techniques.
Traditional alignment techniques fall into three general categories:                1. Mechanical control of the physical alignment errors between the sensor and INU;        2. Measurement of the physical misalignment between the sensor and the INU chassis (e.g., by using a laser); and        3. Iterative measurements of the sensor computed with respect to objects having known locations to determine misalignment.        
For the mechanical control technique, costly and stringent tolerances of the airborne sensor system are maintained during engineering, manufacturing and maintenance. There are practical physical limitations to the accuracy because the mechanical alignment is subject to tolerance error stack-up (e.g., adding up the mechanical tolerance, or misalignment) between the sensor antenna and the mounting point of the antenna on its fuselage (i.e., the fuselage and the INU). A variation of the mechanical control scheme includes a second INU mounted on the sensor, but it is subject to similar limitations and error stack-up.
The second technique (measurement of the physical misalignment) involves measurements of distances between the sensor and INU using lasers and/or mirrors for precision angle measurements. This technique is costly and time-consuming because the INU is carried inside the aircraft's fuselage and the sensor antenna is mounted external to the fuselage, so the measurement set-up is relatively complex because the observations are not direct.
Iterative measurement, the third technique, involves using the installed sensor to locate objects that have known geolocations. The observed geolocation errors are then used to “back out” the misalignment. Because the geolocation errors are driven by several factors, including the misalignment in more than one axis (e.g., pitch, roll and yaw), this is an iterative and time-consuming process, and especially if airborne operations are required to take the measurements.
There is a need for an alignment technique that achieves greater accuracy without the drawbacks associated with the traditional techniques discussed above.